Stasis in the Smaller Owls from Rancho La Brea During the Last Glacial-Interglacial Climate Change

Palaeontologia Electronica palaeo-electronica.org

Stasis in the smaller owls from Rancho La Brea during the last glacial-interglacial climate change

Meena Madan, Donald R. Prothero, and Valerie J.P. Syverson

ABSTRACT

Conventional evolutionary biology highlights examples like the Galápagos finches, which show rapid responses to climatic change. Previous studies of many common birds of La Brea, including Teratornis merriami, La Brea Condors (Gymno- gyps amplus), Golden Eagles (Aquila chrysaetos), Bald Eagles (Haliaeetus leucocephalus), Californian Turkeys (Meleagris californica), Caracaras (Caracara plancus prelutosus), and Black Vultures (Coragyps occidentalis), as well as the two larger owls, the Great Horned Owl (Bubo virginianus) and the Barn Owl (Tyto alba), showed complete stasis in size and shape through the last glacial-interglacial cycle. Are the smaller birds, especially the small owls, as unresponsive to climate change as the larger birds, or are they more like the Galápagos finches? We measured the large samples of the crow-sized Long-Eared Owl (Asio otus) and the robin-sized Burrowing Owl (Athene cunicularia) from the collections of the La Brea Tar Pits Museum to determine if they showed size or shape changes in response to the climate changes of the last 35,000 years. Even though living Burrowing Owls exhibit a weak Bergmann’s rule effect, with larger subspecies in colder climates, neither species of small owls from Rancho La Brea showed statistically significant changes in size or robustness even during the peak glacial interval at 20,000-18,000 years ago, when the climate at Rancho La Brea was dominated by coniferous forests and snowy winters. Apparently, most birds do not respond to long-term changes in climate in a simple fashion, but are ecologically flexi- ble and live in a wide range of habitats and climates without change in size or limb robustness.

Meena Madan. School of Earth Sciences, University of Bristol, Bristol, BS8 1TQ, UK; [email protected]; Donald R. Prothero. Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, CA 90007; [email protected] (corresponding author) Valerie J.P. Syverson. Dept. Geosciences, Univ. Wisconsin, 1215 W. Dayton St., Madison, WI 53706; [email protected]

Madan, Meena, Prothero, Donald R., and Syverson, Valerie J.P. 2019. Stasis in the smaller owls from Rancho La Brea during the last glacial-interglacial climate change. Palaeontologia Electronica 22.3.70 1-12. https://doi.org/10.26879/960 palaeo-electronica.org/content/2019/2796-stasis-in-pleistocene-owls

Copyright: November 2019 Society of Vertebrate Paleontology. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. creativecommons.org/licenses/by/4.0/creativecommons.org/licenses/by-nc-sa/4.0/ MADAN, PROTHERO, & SYVERSON: STASIS IN PLEISTOCENE OWLS

Keywords: Pleistocene; birds; evolution; punctuated equilibria; climate change; owls

Submission: 14 January 2019. Acceptance: 9 October 2019.

INTRODUCTION poral sequence of most of the pits and their corre- spondence to Late Pleistocene climatic cycles. Conventional evolutionary biology has long The RLB tar pits also preserve the climatic featured examples of adaptive responses to cli- record in southern California at the time they matic change, especially in birds such as the formed, based upon data from snails, pollen, plant Galápagos finches (Weiner, 1995; Grant and macrofossils, and stable oxygen isotopes (Warter, Weiner, 1999; Grant and Grant, 2007). There are 1976; Coltrain et al., 2004; Ward et al., 2005). The numerous other demonstrated instances of micro- best record of this time interval comes from deep- evolutionary change in modern birds, such as sea cores drilled just offshore in the California con- Siberian Warblers (Locustella thoracica), English tinental shelf. Based on pollen grains analyzed by Sparrows (Passer domesticus), Cuckoos (Cuculus Heusser (1998), there was a change from oak and canorus), Cowbirds (Molothrus ater), Red-Winged chaparral vegetation about 59 ka to pine-juniper- Blackbirds (Agelaius phoeniceus), and many oth- cypress woodlands at 24 ka, then to a closed-cone ers (Weiner, 1995). These studies all suggest that juniper-ponderosa forest with abundant winter body size and robustness in birds are highly snow during the last glacial maximum (24-14 ka). responsive to environmental and climatic changes. During the glacial-interglacial transition from 14 to But for 47 years now, paleontologists have 10 ka, the landscape returned to dominant oak- documented the prevalence of morphological sta- chaparral and coastal sagebrush with pulses of sis among fossil populations over long time inter- alder. In the past 10,000 years, the region has vals (Eldredge and Gould, 1972; Eldredge, 1999; been vegetated by the modern assemblage of oak- Gould, 2002). From this perspective, it seems that chaparral-herbaceous vegetation. According to the short-term examples of small-scale change stable oxygen and carbon isotopic analysis (Col- may not be very important to large-scale macro- train et al., 2004), there was increased seasonal evolution. Most fossil metazoans show evolution- aridity during the last interglacial and previous gla- ary stasis over timescales of millions of years cial. (Jackson and Cheetham, 1999; Jablonski, 2000; So how did climatic and vegetational change Gould, 2002; Jablonski, 2008; Geary, 2009; Hal- affect the birds at RLB? We might expect to see lam, 2009; Princehouse, 2009; Sepkoski and changes consistent with Bergmann’s rule of larger Ruse, 2009; Hunt et al., 2015). There is abundant body size in colder climates at the times when evidence of stasis during periods of climatic Rancho La Brea was at its coldest and snowiest change and stress (e.g., Coope, 1979; Davis, about 20-18 ka ago during the last glacial maxi- 1983; Bennett, 1990; Prothero and Heaton, 1996; mum. The most common bird at RLB, the Golden Prothero, 1999; Prothero et al., 2012), which Eagle (Aquila chrysaetos), shows considerable should be intervals of morphological change clinal variation today, with larger-bodied subspe- according to conventional evolutionary biology. cies in the high latitudes in both Siberia and North The Rancho La Brea (RLB) tar pits are a per- America (Brown, 1968; Johnsgard, 1990). Yet fect place to test the hypothesis of short-term Molina and Prothero (2011) analyzed the large change in response to climate on timescales of RLB sample of Golden Eagles and found no indica- thousands of years. The tar pits produce a huge tion of larger body sizes during the peak glacial sample of fossil birds with over 85,000 individual interval at 20-18 ka years ago. Syverson and bones representing at least 133 species, including Prothero (2010) found no size differences over the 19 extinct species (Howard, 1962). Asphalt is par- same period in the third most common bird, the ticularly well suited to trapping and preserving deli- extinct California Condor Gymnogyps amplus. cate bird bones, so there are large samples of Fragomeni and Prothero (2011) found no signifi- many bones from a variety of species and time cant size or robustness changes in the second intervals (Akersten et al., 1983; Stock and Harris, most common bird, the extinct Californian Turkey 1992; Friscia et al., 2008). Many of the pits have (Meleagris californica), nor in the La Brea Caracara also been radiocarbon dated (Marcus and Berger, (Caracara plancus prelutosus), nor in the Bald 1984; O’Keefe et al., 2009), so we know the tem- Eagle (Haliaeetus leucocephalus). Gillespy et al.

2 PALAEO-ELECTRONICA.ORG

(2016) found no size or shape changes in the huge We measured the tarsometatarsus (TMT), extinct condor-like Teratornis merriami. Long et al. which is by far the most robust element in the bird (2016) documented complete size and shape sta- skeleton and is much less likely to be broken or sis in the Black Vultures (Coragyps occidentalis). deformed. Based on a more extensive set of mea- Madan et al. (2015) found stasis in the Great surements in the La Brea condor Gymnogyps Horned Owls (Bubo virginianus), and Madan et al. amplus (Syverson and Prothero, 2010), which (2016) documented the same in the Barn Owls demonstrated no differences between the trends (Tyto alba). These are the most common birds in shown in TMT measurements and those of the the RLB collections, and all of them exhibit mor- other parts of the skeleton, we assume that the phological stasis for the later Pleistocene TMT is a good proxy for within-species body size sequence documented at the RLB. variation, and it has been widely used by ornitholo- However, these are all relatively large birds, gists and paleo-ornithologists for that purpose. most of which live in large ranges and are able to Only complete, undeformed adult TMTs were mea- adapt to a wide variety of habitats. Smaller birds, sured in order to avoid artifacts resulting from which tend to have limited geographic ranges and breakage or ontogeny. narrower habitat preferences, might be more com- We searched the museum’s Excel database parable to the Galápagos finches in their evolution- for all RLB birds by the pit number first in order to ary response to rapid climate change. For this avoid measuring specimens from pits like Pit 16, study, we examined the two common small owls in which has problematic, widely scattered radiocar- the RLB collections. One is the crow-sized Long- bon ages (Marcus and Berger, 1984; O’Keefe et Eared Owl (Asio otus), which does not have a al., 2009), and so could not be used in our study. strong Bergmann’s rule effect in modern popula- For unknown reasons, Pit 16 produces a high per- tions because it migrates from cold climates in the centage of the bird bones from RLB (Howard, winter. The other is the robin-sized Burrowing Owl 1962), but the dating is too poor to be used for (Athene cunicularia), which has numerous named time-series studies like this one. We measured all subspecies, of which the tropical ones are slightly adult TMT specimens from the pits with well-con- smaller than the ones from colder climates (Dun- strained ages. We also measured TMTs of modern can, 2003; König and Weick, 2008). Burrowing Owls from the American Museum of Natural History (AMNH) in New York City, and both METHODS Burrowing Owls and Long-Eared Owls from the Museum of Vertebrate Zoology at the University of We studied the large sample of Long-Eared California (UCMVZ), Berkeley. Owls and Burrowing Owls in the RLB collections. We measured a total of 168 Asio otus and 92 According to Howard (1962, table 1), the minimum Athene cunicularia TMTs with digital metric cali- numbers of individuals in the collection are 177 pers. Three dimensions were measured (Figure 1): Long-Eared Owls and 228 Burrowing Owls. They maximum shaft length between the longest proxi- have been found in almost every pit, including the mal-distal ends of the TMT; midshaft anteroposte- oldest pit, Pit 77 (35 ka).

FIGURE 1. Image of a Long-Eared Owl TMT, showing the measurement landmarks. Abbreviations: ML, maximum shaft length between the longest proximal-distal ends of the TMT; MW, midshaft transverse width of the TMT; TMT, tarsometatarsus.

3 MADAN, PROTHERO, & SYVERSON: STASIS IN PLEISTOCENE OWLS

TABLE 1. Basic statistics of La Brea TMTs of Athene cunicularia. SD: Standard Deviation; CV: Coefficient of Variation; N: sample size; TMT: tarsometatarsus.

Age Mean SD Character (ka) N (mm) (mm) CV Length 29 12 48.60 1.41 2.90 18 26 47.11 1.58 3.35 14 3 49.27 2.25 4.57 9 33 48.09 1.78 3.71 0 17 46.64 3.58 7.68 Midshaft transverse width 29 12 2.85 0.17 6.08 18 26 2.75 0.25 9.23 14 3 2.60 0.10 3.85 9 33 2.77 0.21 7.49 0 17 2.85 0.39 13.83 Midshaft depth (antero-posterior) 29 12 2.71 0.22 8.10 18 26 2.41 0.28 11.51 14 3 2.57 0.12 4.50 9 33 2.38 0.27 11.30 0 17 2.84 0.51 18.07

TABLE 2. Basic statistics of La Brea TMTs of Asio otus. SD: Standard Deviation; CV: Coefficient of Variation; N: sample size; TMT: tarsometatarsus.

Age Mean SD Character (ka) N (mm) (mm) CV Length 29 23 45.26 1.48 3.27 21 1 42.40 NA NA 18 47 44.18 1.65 3.74 16 1 43.20 NA NA 14 11 44.45 1.34 3.02 11 3 45.13 1.44 3.18 9 74 44.40 1.76 3.97 0 8 44.29 1.70 3.84 Midshaft transverse width 29 23 4.71 0.37 7.92 21 1 5.00 NA NA 18 47 4.54 0.29 6.49 16 1 5.20 NA NA 14 11 4.60 0.33 7.14 11 3 5.00 0.10 2.00 9 74 4.60 0.24 5.26 0 8 4.51 0.34 7.63 Midshaft depth (antero-posterior) 29 23 3.36 0.30 9.06 21 1 3.00 NA NA 18 47 3.29 0.33 10.00 16 1 3.60 NA NA 14 11 3.23 0.21 6.36 11 3 3.37 0.21 6.18 9 74 3.23 0.27 8.32 0 8 3.46 0.13 3.76 4 PALAEO-ELECTRONICA.ORG

TABLE 3. Mann-Whitney U tests for each dimension of TMTs for Athene cunicularia, comparing measurements from each time interval to the rest of the pooled samples; jackknife method was used instead of straight pooled variation due to highly uneven sample sizes. Bold face indicates results that are significantly different at p < 0.05 with Bonferroni cor- rection applied. The significant differences given by this test are consistent with the significance levels reported by the Kruskal-Wallis test (df = 4). Corrected significance level: p < 0.01. Depth and robustness measurements were significantly different from pooled samples for the 9 ka and modern samples.

Age Length Width Depth Robustness (ka) UpUpUpUp 29 624.5 0.078 597.5 0.145 692 0.010 639.5 0.053 18 630 0.059 823.5 0.852 652 0.089 733 0.327 14 187.5 0.221 59 0.103 152 0.663 96 0.430 9 1090.5 0.272 905.5 0.670 626 0.006 596.5 0.003 0 504.5 0.207 651.5 0.821 915 0.004 972 0.001

TABLE 4. Mann-Whitney U tests for each dimension of TMTs for Asio otus, comparing measurements from each time interval to the rest of the pooled samples; jackknife method was used instead of straight pooled variation due to highly uneven sample sizes. Corrected significance level: p < 0.00625. No differences were significant by either test.

Age Length Width Depth Robustness (ka) UpUpUpUp 29 2202 0.014 2029.5 0.093 2065 0.065 2018.5 0.106 21 18.5 0.183 152.5 0.155 27 0.244 105 0.665 18 2456 0.171 2293.5 0.051 2919 0.789 2759 0.767 16 41.5 0.392 163 0.101 151 0.163 164 0.099 14 875 0.944 819.5 0.779 736 0.411 776.5 0.579 11 323.5 0.366 450.5 0.015 293 0.587 382 0.109 9 3384 0.765 3456 0.945 2788 0.026 2916 0.073 0 606.5 0.806 542.5 0.467 928 0.031 786 0.279 rior thickness of the TMT; and midshaft transverse RESULTS width of the TMT. These values allowed us to cal- The basic statistics of the two different owl culate a robustness index, the cross-sectional area TMT samples are shown in Tables 1 and 2. The (depth x width) divided by the length, to examine results from the comparisons are reported in shape as well as size. Tables 3 and 4, and shown in Figures 2 to 5. No Once the pit dates had been added to the measurement on any sample was significantly dif- spreadsheet, we performed basic statistical analy- ferently distributed from the remaining specimens sis using Excel for the group of specimens from in the Long-Eared Owl Asio otus, although the each well-dated pit. We then tested the samples for width measurement for the 11 ka group skirted sig- normality using the Shapiro-Wilk method. Since nificance. In the Burrowing Owl Athene cunicularia, none of the pit samples were normally distributed, the 9 ka and 0 ka samples had a significantly differ- we used the nonparametric Kruskal-Wallis test to ent distribution in depth and robustness, but we determine whether any sample was significantly find no evidence of larger body size or increased different from the pooled mean of all other mea- robustness in the samples from around the last gla- surements. The different pit samples were then cial maximum at 20-18 ka. compared to determine which ones were different Evaluating models for the time series (Tables from the pooled sample mean minus that group by 5, 6; Figures 6, 7) confirms the lack of any overall the Mann-Whitney U test. The time series of each directional change over the time period being ana- measurement was also fit to evolutionary models lyzed. The best evolutionary model for most of the (directional random walk, undirected random walk, time series is stasis (i.e., no change in mean stasis, and strict stasis) in R using the paleoTS value), with the exception of the derived robust- package (Hunt, 2014). ness measure in Athene. Indeed, for five of the

5 MADAN, PROTHERO, & SYVERSON: STASIS IN PLEISTOCENE OWLS

AtheneTMT length Asio TMT Robustness

60 0.5 0.45

0.4 50 0.35

0.3

40 0.25 Data 0.20 Means 30 0.15 Data 0.1

Means Midshaft area/length

TMT length (mm) TMT 0.05 20 0 35 30 25 20 15 10 5 0 Age (ka) 10

0 FIGURE 5. Plot of the robustness (midshaft cross-sec- 35 30 25 20 15 10 5 0 Age (ka) tional area divided by length) of Asio otus TMTs through time. Open diamonds: individual specimens; solid FIGURE 2. Plot of the lengths of La Brea Athene cunic- squares: mean for each pit. ularia TMTs through time. Open diamonds: individual specimens; solid squares: mean for each pit. TABLE 5. Time-series analysis, given in Akaike weights, for Athene cunicularia. GRW: general random walk; URW: unidirectional random walk; for “strict stasis” defini- Asio TMT length tion, see Hunt et al. (2015). The favored model for each 60 time series is shown in bold face. Strict stasis was pre- ferred for length and width measurements. Depth mea- 50 surements fit best as ordinary stasis (i.e., true mean not

40 constrained) due to the larger measurements in the 0 ka group. Robustness, derived from the combination of the 30 three measurements, favored an unbiased random walk Data Means model. 20 length (mm) TMT GRW URW Stasis Strict stasis 10 Length 0 0.076 0.146 0.778

0 Width 0 0.055 0.085 0.86 35 30 25 20 15 10 5 0 Age (ka) Depth 0 0.422 0.561 0.017 Robustness 0 0.613 0.353 0.035 FIGURE 3. Plot of the lengths of La Brea Asio otus TMTs through time. Open diamonds: individual specimens; solid squares: mean for each pit. TABLE 6. Time-series analysis, given in Akaike weights, for Asio otus. GRW: general random walk; URW: unidirectional random walk; for “strict stasis” definition, see AtheneTMT Robustness Hunt et al. (2015). The favored model for each time 0.3 series is shown in bold face. All time series best fit a strict

0.25 stasis model except width, which favored ordinary stasis due to the large difference in the 12 ka sample. 0.20 Strict

0.15 GRW URW Stasis stasis Data Means Length 0.003 0.089 0.102 0.807 0.1 Midshaft area/length Width 0.001 0.08 0.919 0 0.05 Depth 0.003 0.35 0.56 0.088

0 Robustness 0 0.068 0.107 0.825 35 30 25 20 15 10 5 0 Age (ka) FIGURE 4. Plot of the robustness (midshaft cross-sectional area divided by length) of Athene cunicularia TMTs through time. Open diamonds: individual specimens; solid squares: mean for each pit.

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FIGURE 6. Time-series analysis of length (mm), width (mm), depth (mm), and robustness of tarsometarsi of Asio otus through the last 29 ka at RLB. As is apparent from the statistical analysis (Table 6), they exhibit either a unidirectional random walk or complete stasis.

eight time series modeled, the best model is strict ary model fit results makes it apparent that the 0 ka stasis, which is defined in the paleoTS implementa- sample of Athene has unusually short and deep tion as a stasis model with zero variance around (anteroposteriorly thick) TMTs, which is the likely the long-term mean. The exceptions are depth and reason why strict stasis does not model these time robustness in Athene and width in Asio. Comparing series well (but see below). The Asio width result the summary statistics (Table 1) with the evolution- appears to be due entirely to the small (n = 3) and

FIGURE 7. Time-series analysis of length (mm), width (mm), depth (mm), and robustness of tarsometarsi of Athene cunicularia through the last 29 ka at RLB. As is apparent from the statistical analysis (Table 5), they exhibit either a unidirectional random walk or complete stasis.

7 MADAN, PROTHERO, & SYVERSON: STASIS IN PLEISTOCENE OWLS

anomalously wide 11 ka sample. No trend of even though many of these species ranged greater size or robustness occurs around the 18 ka through several glacial-interglacial cycles. Thus, sample at the peak of the glaciation in either spe- stasis in body size is a widespread phenomenon in cies. nearly all Pleistocene birds and mammals over the In consideration of the fact that the 0 ka sam- entire span of several glacial-interglacial cycles. ple of Burrowing Owls consisted of samples from Paleontologists and neontologists have long two institutions on opposite sides of North America, argued about the significance of stasis despite which could potentially introduce a confounding changes in environment. It is clearly inconsistent geographic signal, we also analyzed a reduced with the notion of adaptive sensitivity shown by the data set containing only the UCMVZ and RLB Galápagos finches and other recently documented specimens and excluding those from AMNH. examples of adaptation and microevolution on Although the AMNH and UCMVZ specimens were short time scales (Weiner, 1995). Ideas like stabi- not significantly different in any dimension (two- lizing selection (Estes and Arnold, 2007) are tailed t-test, significance threshold p < 0.05), their clearly inapplicable and fail to explain this phenom- distributions were not identical. The reduced data enon, since the environment in this case is chang- set displayed some minor differences from the full ing, not stabilizing (Lieberman and Dudgeon, data set: it had no significant difference in robust- 1996). Other ideas, such as developmental canali- ness, but the 29 ka sample emerged as signifi- zation, have been used to explain this stability, but cantly larger in the depth dimension. Most of the this model has fallen out of favor in view of the phe- short and deep TMTs in the 0 ka sample were notypic plasticity of domesticated animals, such as AMNH specimens. Therefore, the time series dogs (Gould, 2002; Eldredge et al., 2005). Bennett model fit results with the reduced data set were (1990, 1997) argued that the climate changes of similar to those with the full data set, except for the the Pleistocene were too rapid for organisms to robustness time series, which without the AMNH respond, but the Pleistocene fossil record spans specimens favored a strict stasis model over a ran- tens to hundreds of thousands of years. If the dom walk. Galápagos finches could show change in just a few years, then evolutionary changes could occur in a DISCUSSION matter of years or decades. Although it has been suggested that mean phenotype fluctuates on a There is no evidence of significant size or time scale rapid enough to appear static in the fos- shape changes in the small owls across the last sil record, most RLB pits have narrow enough time glacial maximum despite the dramatic changes in constraints that they should capture changes on climate and vegetation documented in the region this time scale. over the past 40,000 years, consistent with the pre- However, recent studies using large data- vious analyses of all the common larger birds at bases of evolutionary time series indicate that such RLB. As is the case with Golden Eagles (Molina classic examples of rapid change, rather than con- and Prothero, 2011), Bald Eagles and Caracaras stituting true evolutionary shifts in the species’ (Fragomeni and Prothero, 2011), Great Horned adaptive zone, more likely result from short-term, Owls (Madan et al., 2015), and Barn Owls (Madan reversible shifts within the species’ existing adap- et al., 2016), modern Burrowing Owl subspecies tive zone. Uyeda et al. (2011) looked at data from exhibit clinal variation in size over latitude following short time scales of historic data up to million-year Bergmann’s rule (Duncan, 2003; König and Weick, time scales. They found that rapid short-term evo- 2008). Yet at RLB, even conditions of coniferous lution on timescales of decades to centuries are forests and frequent snow at low elevations around highly constrained (“bounded fluctuations”) and do 20-18 ka did not cause measurable increases in not accumulate change over time or produce major body size in any of these birds. This trend is also adaptive shifts or new species. Instead, rare bursts consistent with nearly all the larger mammals of of change that accumulate over million-year times- RLB, which also show no response in size or cales account for most of the morphological shape during the maximum climatic changes of the changes and speciation within lineages, strongly past 35,000 years (Prothero and Raymond, 2008; confirming the original punctuated equilibria model DeSantis et al., 2011; Madan et al., 2011; Prothero of Eldredge and Gould (1972). Hunt et al. (2015) and Raymond, 2011; Raymond and Prothero, similarly found that 38 % of paleontological time 2011; Prothero et al., 2012). Numerous studies series were best fit by a stasis model across a wide (Barnosky, 1994, 2005) document stasis in body range of taxa and temporal resolutions, but espe- size in nearly every Pleistocene mammal lineage,

8 PALAEO-ELECTRONICA.ORG

cially for time series driven by sub-million-year fluc- strong tendency in many of these species to follow tuations in global temperature. Bergmann’s rule (Syverson and Prothero, 2010; The most widely accepted explanation for Fragomeni and Prothero, 2011; Molina and Proth- long-term stasis is the idea that most organisms ero, 2011; Madan et al., 2015; Gillespy et al., 2016; that have large geographic ranges are also Madan et al., 2016). This agrees with the evidence adapted to a wide spectrum of local environments, that all RLB mammals and birds with sufficient so they do not respond to environmental change by sample sizes also show complete stasis across the means of morphological change (Lieberman et al., major climatic shifts in and out of the last glacial 1995; Lieberman and Dudgeon, 1996; Eldredge, maximum. This phenomenon seems to be 1999; Eldredge et al., 2005). This might be appro- explained by recent model-fitting results, which priate for large birds that range all over the Ameri- confirm that short-term morphological changes are cas, but not for smaller birds, some of which live in not cumulative, but concentrated in rapid bursts of very small areas. However, both the owl species in evolution and speciation observable over long mil- this study, including the small and sedentary Bur- lion-year time scales, followed by long-term stasis rowing Owl, are just as static in the face of climate (Uyeda et al., 2011). Although the stasis exhibited change as the larger birds of RLB. Most species of by larger mammals and birds might be explained Pleistocene small mammals also remain static in by wide geographic ranges and environmental flex- size through changing climate regimes (Barnosky, ibility, this model does not yet explain why environ- 2005). Such stasis in small mammal populations is mentally restricted, nonmigratory mammals and also apparent during late Eocene-Oligocene cli- birds with small body sizes and home ranges also mate changes (Prothero and Heaton, 1996). What- demonstrate stasis. ever the mechanism may be that maintains stasis in body size over this range of timescales, it does ACKNOWLEDGMENTS not appear to be contingent on a species’ size or its We thank A. Farrell, J. Harris, and especially consequent range of environmental tolerance. K. Campbell, Jr., for allowing access to the La Brea Tar Pits Museum collection of birds. We thank C. CONCLUSIONS Cicero of the University of California Museum of Statistical analysis of size and shape variables Vertebrate Zoology, and P. Capainolo, L. Garetano, of the Long-Eared Owls and Burrowing Owls of and J. Cracraft of the American Museum of Natural RLB demonstrates almost no significant size or History Department of Ornithology for allowing us robustness change in response to the dramatic to measure modern birds in their collections. We cooling and vegetational change of the peak glacial thank two anonymous reviewers for their helpful period 20,000 years ago or the subsequent warm- comments on the manuscript. DRP is grateful to ing. This is consistent with previous studies show- the late L. Marcus for teaching him biometrics and ing stasis in Golden Eagles, Bald Eagles, Condors, for introducing him to the wonders of Rancho La Turkeys, Caracaras, Teratorns, Black Vultures, Brea. Great Horned Owls, and Barn Owls, despite their

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